Modular communication systems, such as those used in spaceborne and airborne applications, typically employ highly compact and densified signal transmission/feed networks, often configured as multilayer stripline architectures, to interconnect various components, such as RF signal processing (amplifier and impedance/phase control) circuits and beam-forming circuits, for a phased array antenna. To minimize size and weight it is common practice to stack multiple layers of microstrip conductor lines, or stripline, closely together as a laminated arrangement of printed circuits. A simplified illustration of such a stripline structure is diagrammatically illustrated in FIGS. 1 and 2 as patterns of conductors 1 and 2 and intermediate dielectric layers 3 (see FIG. 1), that are stacked together to form a multilayer signal transmission architecture.
Because high frequency signal distribution networks, such as those employed for (RF) signalling applications in the hundreds of MHz or into the high (several to tens and above) GHz range, readily couple (radiate and receive) substantial electromagnetic energy in addition to that of the signals propagating through the conductors of the networks, it is necessary to carefully configure and/or space such networks with respect to one another and adjacent system components. In FIGS. 1 and 2, this internal separation is shown by a horizontal spacing 4 and a vertical spacing corresponding to the thickness of a dielectric layer 3 between respective ones of the conductors 1 and 2. As far as the external environment is concerned, the unwanted signal coupling problem is addressed by the use of (grounded) shielding layers, shown at 5 and 6 in FIG. 1.
However, within the multilayer structure itself it can be expected that conductors of the respective networks will cross over or overlap one another at one or more locations, one of which is shown at 7 in FIG. 2. Because of the relatively reduced vertical separation between the conductors of the respective layers of the laminate, unwanted mutual coupling or cross-talk between the networks will occur at these cross-over points. A customary practice to solve this cross-talk problem is diagrammatically illustrated in FIG. 3, and involves the insertion of an interposed shielding (grounded) layer 8, such as a layer of copper or the like (and an associated dielectric layer 3) between each stripline transmission layer. The dielectric shielding layer is shown as dielectrically isolated from conductor layers 1 and 2 by dielectric layers 3 therebetween. Outer grounded shielding layers shown at 5 and 6 are respectively disclosed atop and beneath the conductors 1 and 2 by means of dielectric layers 3 therebetween.
Unfortunately, these additional shielding and dielectric layers not only add weight but substantially increase the overall thickness of the laminate. This creates the need for a trade-off between the thickness of the dielectric layers and the lossiness of the stripline. Namely, because the effective impedance of the stripline is dependent upon its proximity to a ground layer, in order to maintain a desired characteristic line impedance (e.g., fifty ohms, nominal) it is necessary to reduce the line width of the stripline as the thickness of the dielectric layer is decreased. However, narrowing the stripline increases its resistance and therefore its `lossiness`.